1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer in which an electrophotographic process is adopted. More particularly the invention relates to the image forming apparatus including image forming means for forming an unfixed toner image in a recording medium and heating and fixing means for heating and fixing the unfixed toner image to the recording medium.
2. Related Background Art
In the image forming apparatus, generally heating and fixing means (hereinafter referred to as fixing device) includes electric heat generation means (heat source) for having an electric heat generation member generating heat(corresponding to heat dissipation amount) by electrifying, a power supply which supplies current to the electric heat generation member, temperature detection means for detecting a temperature of the electric heat generation member, and control means for controlling supply current from the power supply to the electric heat generation member based on a signal from the temperature detection means.
(Unitization Configuration of Fixing Device)
In the electric heat generation member of the fixing device, an appropriate temperature and a heat distribution are required in order to secure desired fixing performance. Therefore, the electric heat generation members having different specifications are required depending on a rated supply voltage of the image forming apparatus. Specifically, the electric heat generation member having a small resistance value is required for a main body of the image forming apparatus whose rated supply voltage is 100V, and the electric heat generation member having a large resistance value is required for the main body of the image forming apparatus whose rated supply voltage is 200V.
Accordingly, usually the fixing device is unitized and the fixing device is adapted to be attachable to and detachable from the main body of the image forming apparatus. In the case of the unitized fixing device, as disclosed in Japanese Patent Application Laid-Open No. 2003-50522, a method of connecting the fixing device and the main body of the image forming apparatus with a connector is widely adopted.
(The Conventional Technologies for Anomalous Overheating Avoidance)
In the fixing device, an image fixing temperature is controlled at a predetermined temperature for image fixing by the above configuration. When any one of the electric heat generation member, the power supply, the temperature detection means, and the control means malfunctions in the fixing device, the fixing device does not work as the fixing device. Further, when electrifying runaway is generated, there is a fear that the fixing device overheats.
Therefore, in the fixing device, as disclosed in Japanese Patent Application Laid-Open No. H08-248813, the temperature detection means such as a thermistor is arranged in the electric heat generation means. When the fixing device is in an overheating state, electrifying the electric heat generation member is cut off by current cut-off means such as a relay inserted into an electrifying circuit.
FIG. 7 shows a configuration of the conventional overheating avoidance device in the image forming apparatus. The reference numeral 1001 denotes a main body of the image forming apparatus, and the reference numeral 1002 denotes a fixing device attachable to and detachable from the image forming apparatus 1001. The image forming apparatus 1001 and the fixing device 1002 are electrically connected to each other through connectors 1006 and 1007. An AC power supply 1005 is connected to an electric heat generation member 1003 through the connector 1006. The electric heat generation member 1003 is electric heat dissipating means on the fixing device 1002 side. On the other hand, a thermistor 1004 which is of temperature detecting means is connected through the connector 1007. The thermistor 1004 is arranged near the electric heat generation member 1003 which is of the electric heat dissipating means. The output voltage of the thermistor 1004 is divided by a resistor 1014 and input to an operational amplifier 1011. The divided voltage level of resistors 1012 and 1013 is also input to the operational amplifier 1011, and the divided voltage level is compared to the output level of the thermistor 1004. When the thermistor 1004 is higher than a predetermined temperature, a relay 1009 is cut off to stop the power supply to the electric heat generation member 1003.
(Conventional Technologies for Fixing-Device Specifications Identification)
In the image forming apparatus to which the unitized fixing device is attached, mismatching is generated when the heat generation member is attached to the image forming apparatus while the rated supply voltage of the image forming apparatus and the heat generation member specification of the fixing device are wrongly combined. In order to avoid the mismatching, Japanese Patent Application Laid-Open No. H11-84943 discloses a mode in which rated voltage identifying means is provided in the fixing device. In the system, when the mismatching is generated between the fixing device and the image forming apparatus, electrifying the electric heat generation member is stopped.
Referring to FIG. 7, the conventional rated voltage identifying mode will be described. The method of connecting first and second terminals of the connector 1007 on the fixing device side is set according to the rated supply voltage. The second terminal is connected to a ground GND on the image forming apparatus 1001 side. On the other hand, the first terminal is pulled-up by a resistor 1016 and connected to an input port of a CPU 1017. When the rated supply voltage of the fixing device 1002 is 100V, the CPU 1017 can detect the rated supply voltage of the fixing device 1002 by connecting the first terminal and the second terminal to each other with a jumper cable 1008. When the rated supply voltage is 200V, the CPU 1017 can detect the rated supply voltage of the fixing device 1002 by not connecting the first terminal and the second terminal to each other with the jumper cable 1008. At this point, when the CPU determined that the fixing device 1002 differs from the image forming apparatus 1001 in the rated supply voltage, the heat generation member is not electrified by turning off a triac 1010.
However, in the image forming apparatus in which the unitized fixing device described above is electrically connected to the main body of the image forming apparatus with the connectors 1006 and 1007, the overheating avoiding means and the fixing device specifications identifying means are not normally operated when the control-system connector 1007 is not connected while the power-supply-system connector 1006 is in the connected state (fitted state). When the power-supply-system connector 1006 is not connected, the overheating is not generated because the electric power is not supplied to the fixing device side.
The action when the control-system connector 1007 is in the disconnected state while the power supply-system-connector 1006 is in the connected state (fitted state) in the image forming apparatus having the configuration of FIG. 7 will be described below. When a fourth terminal for thermistor detection signal which is of a temperature detection signal is not connected by the disconnection of the connector 1007, the CPU becomes a Vcc level by the pull-up of the resistor 1014. Because the thermistor 1004 has the signal level of the low temperature state, software control of the CPU 1017 causes the triac to be electrified, resulting the overheating state of the electric heat generation member 1003. Further, the relay 1009 does not become the cut-off state. Accordingly, the overheating cannot be avoided. When the first and second terminals of the rated voltage identifying means is not connected, because the first and second terminals become the Vcc level by resistors 1015 and 1016, there is a fear that the CPU 1017 cannot identify the rated voltage to generate the overheating.
Although Japanese Patent Application Laid-Open No. H11-344898 teaches a conventional type in those kinds of circuits, a safety circuit having a simpler and safer circuitry than those is desired.